Sepsis is a complex and dynamic disease process that is characterized by activation of the innate immune system, and an inflammatory response to a systemic infection. While an exaggerated immune response has been associated with the morbidity and mortality of severe sepsis and septic shock, anti-inflammatory therapies have not resulted in improvements in patient outcome. Thus, it is important to consider other mechanisms of the host immune response that may be contributing to the detrimental consequences of sepsis. During sepsis, part of the pathobiology of the disease is related to abnormal neutrophil function, and a release of immature forms of neutrophils from the bone marrow, which may contribute to an impaired eradication of infecting microorganisms. As mortality remains high, new and innovative strategies are necessary to advance the treatment of sepsis. Due to the intricacies of the immune response during sepsis, alternative strategies have been considered, including cell-based therapies. Recently, the therapeutic effects of mesenchymal stromal cells (MSCs) have been studied in animal models of sepsis. While the initial studies have looked promising, the mechanisms responsible for the beneficial effects of MSCs have not been fully elucidated. In the present application, we hypothesize that stromal-cell derived factor-1 (SDF-1/CXCL12) is a critical mediator of the MSC response during sepsis, and that MSCs will rescue dysfunctional neutrophils in the setting of sepsis, contributing to an improved outcome. To test our hypotheses, we propose three aims. In Aim 1 we will investigate the importance of SDF-1 for the therapeutic effects of MSCs in an experimental mouse model of sepsis, using cecal ligation and puncture (CLP). Our preliminary data show that treatment with MSCs improves survival and enhances bacterial clearance, and neutrophils are critical for this beneficial response. To test the importance of SDF-1 in MSCs, we will silence SDF-1 and determine whether the protective properties of MSCs are lost in septic animals, compared with wild-type MSCs. Moreover, we will assess the effects of SDF-1 overexpression to improve MSC function. In Aim 2 we will explore the interaction between MSCs and neutrophils during experimental sepsis in mice, and elucidate the role of MSC-derived SDF-1 to improve neutrophil function. We will investigate the ability of MSCs to promote neutrophil maturation, and to improve migration, phagocytosis and bacterial killing. In addition, we will determine whether MSCs alter neutrophil survival, and promote the resolution of inflammation. In Aim 3 we will determine whether MSCs can rescue human neutrophil dysfunction in cells harvested from patients with sepsis, and elucidate the role of SDF-1 in this MSC response. We believe that our application will further reveal the potential of MSCs as a therapy for sepsis, and advance our understanding of how MSCs promote a beneficial response during sepsis.